1. Field of the Invention
The present invention relates to a controller and a method for adaptively adjusting motor driving current waveforms.
2. Description of the Related Art
A common application for motors is to drive a fan. In driving a fan, the driving current waveform of the motor will affect the stability, noise and efficiency of the fan. Typically, a HALL sensor is used in the fan control system; the HALL sensor outputs a HALL signal, based on which a PWM driving signal is generated to control a motor driver circuit, and the latter controls the operation of the motor in response to the PWM driving signal.
Conventionally, when the motor driving current waveform is not good such that the fan has a low operation efficiency, there are two methods to solve the issue. In the first method, the physical location of the HALL sensor is fine-tuned to adjust the control signal of the driver circuit. In the second method, the PWM driving signal is adjusted by hardware circuitry so that the motor driving current waveform is tuned better.
Referring to FIG. 1, in the first method, the level switching points of the PWM driving signal exactly follow the rising and falling edges of the HALL signal (as shown in the figure, the rising edge P1 and falling edge P2 of the PWM driving signal are at the same time as the rising edge H1 and falling edge H2 of the HALL signal). Therefore, this method requires adjusting the physical location of the HALL sensor so as to change the level switching points of the PWM driving signal. However, different fans have different characteristics; this method has to fine-tune the location for every different fan, which is time-consuming and cost-ineffective. Moreover, in this method, the location of the HALL sensor is decided according to the motor driving current waveform when the motor is in full speed operation, but when the fan operates in low speed, the current will be too high and the operation efficiency is low.
Referring to FIG. 2 which is a figure cited from U.S. Pat. No. 7,030,584, in the second method, the PWM driving signal is adjusted by hardware circuitry. In this prior art, the turned-OFF time of the PWM driving signal is pulled in (advanced OFF time) to generate the signal 44 shown in the figure. However, such arrangement is not good enough to completely solve the issue of bad motor driving current waveform. Referring to FIG. 3, when a PWM driving signal is generated according to the positive and negative cycles of a HALL signal, and the PWM driving signal is turned OFF early, an imbalance occurs in the motor driving current waveform (shown by the references C1 and C2). Moreover, for a different fan, the hardware circuitry needs to be modified correspondingly, and such modification is time-consuming and cost-ineffective.
In addition to the foregoing drawbacks, when different HALL signals result in different motor driving current waveforms (such as the imbalance in the motor driving current waveform described above, or different motor driving current waveforms caused by different fans), because the location of the HALL sensor or the switching mode in the circuitry of the driver circuit is fixed, the switching points of the driver circuit can not adaptively change in response to different HALL signals. Unless the hardware is modified, the instable fan operation due to imbalanced motor driving current waveform can not be improved.